The present invention relates generally to molds suitable for use in forming cube corner retroreflective sheeting formed from a plurality of laminae, to methods for making such molds, and to retroreflective sheeting formed from such molds which utilize sacrificial laminae to change the configuration of the structured surface in a desired manner.
Retroreflective materials are characterized by redirecting incident light back toward the originating light source. This property has led to the wide-spread use of retroreflective sheeting in a variety of conspicuity applications. Retroreflective sheeting is frequently applied to flat, rigid articles such as, for example, road signs and barricades; however, it is also used on irregular or flexible surfaces. For example, retroreflective sheeting can be adhered to the side of a truck trailer, which requires the sheeting to cover corrugations and protruding rivets, or the sheeting can be adhered to a flexible body portion such as a road worker""s safety vest or other such safety garment. In situations where the underlying surface is irregular or flexible, the retroreflective sheeting desirably possesses the ability to conform to the underlying surface without sacrificing retroreflective performance. Additionally, retroreflective sheeting is frequently packaged and shipped in roll form, thus requiring the sheeting to be sufficiently flexible to be rolled around a core.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as xe2x80x9cbeadedxe2x80x9d sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult). Advantageously, microsphere-based sheeting can generally be adhered to corrugated or flexible surfaces. Also, due to the symmetry of beaded retroreflectors, microsphere-based sheeting exhibits a relatively orientationally uniform total light return when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar base surface and a structured surface comprising a plurality of cube corner elements opposite the base surface. Each cube-corner element comprises three mutually substantially perpendicular optical faces that intersect at a single reference point, or apex. The base of the cube corner element acts as an aperture through which light is transmitted into the cube corner element. In use, light incident on the base surface of the sheeting is refracted at the base surface of the sheeting, transmitted through the bases of the cube corner elements disposed on the sheeting, reflected from each of the three perpendicular cube-corner optical faces, and redirected toward the light source. The symmetry axis, also called the optical axis, of a cube corner element is the axis that extends through the cube corner apex and forms an equal angle with the three optical surfaces of the cube corner element. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the base of the element roughly along the optical axis. The amount of light retroreflected by a cube corner retroreflector drops as the incidence angle deviates from the optical axis.
The maximum retroreflective efficiency of cube corner retroreflective sheeting is a function of the geometry of the cube corner elements on the structured surface of the sheeting. The terms xe2x80x98optically active areaxe2x80x99 and xe2x80x98effective aperturexe2x80x99 are used in the cube corner arts to characterize the portion of a cube corner element that retroreflects light incident on the base of the element. A detailed teaching regarding the determination of the active aperture for a cube corner element design is beyond the scope of the present disclosure. One procedure for determining the effective aperture of a cube corner geometry is presented in Eckhardt, Applied Optics, v. 10, n. Jul. 7, 1971, pp. 1559-1566. U.S. Pat. No. 835,648 to Straubel also discusses the concept of effective aperture. At a given incidence angle, the optically active area can be determined by the topological intersection of the projection of the three cube corner faces onto a plane normal to the refracted incident light with the projection of the image surfaces for the third reflections onto the same plane. The term xe2x80x98percent active areaxe2x80x99 is then defined as the active area divided by the total area of the projection of the cube corner faces. The retroreflective efficiency of retroreflective sheeting correlates directly to this percent active area.
Additionally, the optical characteristics of the retroreflection pattern of retroreflective sheeting are, in part, a function of the physical geometry of the cube corner elements. Thus, distortions in the geometry of the cube corner elements can cause corresponding distortions in the optical characteristics of the sheeting. To inhibit undesirable physical deformation, cube corner elements of retroreflective sheeting are typically made from a material having a relatively high elastic modulus sufficient to inhibit the physical distortion of the cube corner elements during flexing or elastomeric stretching of the sheeting. As discussed above, it is frequently desirable that retroreflective sheeting be sufficiently flexible to allow the sheeting to adhere to a substrate that is corrugated or that is itself flexible, or to allow the retroreflective sheeting to be wound into a roll to facilitate storage and shipping.
Cube corner retroreflective sheeting has traditionally been manufactured by first manufacturing a master mold that includes an image, either negative or positive, of a desired cube corner element geometry. The mold can be replicated using nickel electroplating, chemical vapor deposition or physical vapor deposition to produce tooling for forming cube corner retroreflective sheeting. U.S. Pat. No. 5,156,863 to Pricone, et al. provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and laminate techniques. Each of these techniques has benefits and limitations.
In pin bundling techniques, a plurality of pins, each having a geometric shape on one end, are assembled together to form a cube-corner retroreflective surface. U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat. No. 3,926,402 (Heenan), U.S. Pat. No. 3,541,606 (Heenan et al.) and U.S. Pat. No. 3,632,695 (Howell) provide illustrative examples. Pin bundling techniques offer the ability to manufacture a wide variety of cube corner geometries in a single mold. However, these techniques are economically and technically impractical for making small cube corner elements (e.g. less than about 1.0 millimeters).
In direct machining techniques, a series of grooves are formed in a unitary substrate to yield a cube-corner retroreflective surface. U.S. Pat. No. 3,712,706 (Stamm) and U.S. Pat. No. 4,588,258 (Hoopman) provide illustrative examples. Direct machining techniques can accurately produce very small cube corner elements (e.g. less than about 1.0 millimeters) which is desirable for producing a flexible retroreflective sheeting. However, it is not presently possible to produce certain cube corner geometries that have very high effective apertures at low entrance angles using direct machining techniques. By way of example, the maximum theoretical total light return of the cube corner element geometry depicted in U.S. Pat. No. 3,712,706 is approximately 67%.
In laminate techniques, a plurality of laminae, each lamina having geometric shapes on one end, are assembled together to form a cube-corner retroreflective surface. German Provisional Publication (OS) 19 17 292, International Publication Nos. WO 94/18581 (Bohn, et al.), WO 97/04939 (Mimura et al.), and WO 97/04940 (Mimura et al.), all disclose a molded reflector wherein a grooved surface is formed on a plurality of plates. The plates are then tilted by a certain angle and each second plate is shifted crosswise. This process results in a plurality of cube corner elements, each element formed by two machined surfaces and one side surface of a plate. German Patent DE 42 36 799 to Gubela discloses a method for producing a molding tool with a cubical surface for the production of high-efficiency cube corners. An oblique surface is ground or cut in a first direction over the entire length of one edge of a band. A plurality of notches are then formed in a second direction to form cube corner reflectors on the band. Finally, a plurality of notches are formed vertically in the sides of the band. German Provisional Patent 44 10 994 C2 to Gubela is a related patent.
Cube corner retroreflective sheeting is typically constructed from a substantially optically transmissive polymer base sheet having a substantially planar front surface and a plurality of cube corner elements on its back surface. The sheeting also typically includes a backing sheet that has a suitable adhesive or other means for attaching the sheeting to a desired object.
The term xe2x80x98entrance angularityxe2x80x99 is used in the retroreflective arts to describe the retroreflective efficiency of the sheeting as a function of the entrance angle of incident light, such as described in ASTM E808-94 Standard Practice for Describing Retroreflection. The entrance angularity of cube corner retroreflective sheeting is typically characterized as a function of the entrance angle of incident light measured from an axis normal to the planar surface of the sheeting and as a function of the orientation of the sheeting.
There is typically a trade-off between enhancing the entrance angularity of cube corner retroreflective sheeting at high entrance angles and the retroreflective efficiency at low entrance angles. Thus, cube corner sheeting that has been modified to enhance its entrance angularity typically suffers a degradation in its retroreflective performance in response to light incident on the sheeting at entrance angles less than about 10 to about 20 degrees from an axis normal to the sheeting. This degradation reduces the utility of the retroreflective sheeting for some applications. Thus, there is a need in the art for retroreflective sheeting that exhibits strong entrance angularity performance without suffering significant degradation in retroreflective efficiency at low entrance angles, and for manufacturing techniques for making such retroreflective sheeting.
Preferred retroreflective sheeting disclosed herein has an enhanced entrance angularity without a significant reduction in retroreflective efficiency for light incident on the sheeting at relatively low entrance angles. Also disclosed are preferred methods for manufacturing such retroreflective sheeting including methods for making a master mold suitable for use in forming retroreflective sheeting from a plurality of laminae. Advantageously, master molds manufactured according to these methods enable the manufacture of retroreflective cube corner sheeting that exhibits retroreflective efficiency levels approaching 100% in response to light incident on the sheeting at relatively low entrance angles. Preferred manufacturing methods enable the manufacture of cube corner retroreflective elements having a width of about 0.050 millimeters to about 0.25 millimeters for flexibility of the sheeting. Efficient, cost-effective methods of making molds formed from a plurality of laminae are also disclosed.
In one embodiment, a method is disclosed for manufacturing a plurality of laminae for use in a mold suitable for use in forming retroreflective cube corner articles. Each lamina has opposing first and second major surfaces defining therebetween a first reference plane. Each lamina further includes a working surface connecting the first and second major surfaces. The working surface defines a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane. A third reference plane is defined perpendicular to the first reference plane and the second reference plane. The method includes orienting a plurality of laminae in an assembly to have their respective first reference planes parallel to each other and disposed at a first angle relative to a first reference axis. A plurality of cube corner elements are formed on the working surfaces of the plurality of the laminae, wherein the plurality of cube corner elements have three approximately mutually perpendicular lateral faces that mutually intersect to define a cube corner element. A portion of the laminae are removed from the assembly to alter the configuration of the mold surface.
The step of forming a plurality of cube corner elements on the mold surface comprises forming two secondary groove sets that traverse the working surfaces of the plurality of laminae and that intersect to define a first base angle xcex21 of the respective cube corner elements. A primary groove set is formed that extends between the major surfaces of the respective laminae and that intersect grooves of the secondary groove sets to define a second base angle xcex22 and a third base angle xcex23 of the respective cube corner elements. In one embodiment, a plurality of laminae are removed from the assembly to alter the configuration of the mold surface prior to forming the primary groove set.
In an alternate embodiment where the first angle is greater than zero, the cube corner elements can be formed with two groove sets. The step of forming a plurality of cube corner elements on the mold surface comprises forming a first groove set including at least two parallel adjacent V-shaped grooves in the working surface of each of the laminae. Each of the adjacent grooves defines a first groove surface and a second groove surface that intersect substantially orthogonally to form a first reference edge on each of the respective laminae. A second groove set is formed including at least one groove in the working surfaces of the plurality of laminae. Each groove in the second groove set defines a third groove surface that intersects substantially orthogonally with the first and second groove surfaces to form at least one first cube corner element. The plurality of lamina preferably are oriented to have their respective first reference planes parallel to each other and disposed at a second angle relative to the fixed reference axis prior to forming the second groove set. A plurality of the laminae can be rotated 180xc2x0 about an axis perpendicular to the second reference plane. The step of removing a portion of the laminae can include removing a plurality of the laminae from the assembly and/or machining at least one major surface of a lamina.
One or more of the laminae can be rotated within the assembly 180xc2x0 whereby the working surface is generally in the same plane as adjacent working surfaces. The laminae can be oriented to have their respective first reference planes parallel to each other and disposed at a first angle relative to a fixed reference axis, the plurality of laminae being held in a suitable fixture, the fixture defining a base plane. The fixed reference axis is substantially normal to the base plane. In one embodiment, the first angle measures 0xc2x0. In an alternate embodiment, the first angle measures between about 0xc2x0 and about 40xc2x0, and more preferably from about 7xc2x0 to about 30xc2x0.
In another embodiment, the plurality of laminae are oriented to have their respective first reference planes parallel to each other and disposed at a first angle that is greater than zero during the step of forming the plurality of cube corner elements. The plurality of laminae are reoriented after the step of forming the plurality of cube corner elements to have their respective first reference planes parallel to each other and disposed at a second angle different the first angle. The second angle is preferably less than the first angle, and most preferably zero.
The step of forming the plurality of cube corner elements comprises removing portions of the plurality of laminae proximate the working surface of the plurality of laminae. The step of removing portions of each of the plurality of laminae comprises a material removal technique selected from the group consisting of ruling, fly-cutting, grinding and milling. The adjacent grooves in a groove set can be at different depths in the working surface of the laminae. The distance between adjacent grooves in a groove set can be varied in the working surface of the laminae. A planar interface between the major surfaces is preferably maintained between adjacent laminae during the machining phase and in the subsequent mold formed therefrom so as to minimize alignment problems and damage due to handling of the laminae.
The first base angle xcex21 measures between about 0xc2x0 and about 60xc2x0, and more preferably between about 10xc2x0 and about 45xc2x0, and most preferably between about 24xc2x0 and about 40xc2x0. The primary groove set can be formed in every nth laminae, where n is a number greater than 1. The cube corner elements are typically arranged in opposing pairs. In an alternate embodiment, optical axes of the cube corner elements can be generally parallel to provide an asymmetrical total light return about a 360xc2x0 range of orientation angles.
The step of removing a plurality of the laminae comprises removing laminae selected from the group of laminae which lack a primary groove in their respective working surfaces. The method also includes reassembling a plurality of the laminae comprising a primary groove on their respective working surfaces to form a mold surface including a plurality of cube corner element segments. The cube corner element segments preferably represent more optically active portions of the respective cube corner elements.
The present method also preferably includes replicating the working surface of the mold to form a negative copy of the plurality of cube corner elements suitable for use as a mold for forming retroreflective articles. Also disclosed is a mold comprising a negative copy of the plurality of cube corner elements manufactured according to the present method. Also disclosed is a method of forming a retroreflective sheeting comprising providing the preferred mold and forming a retroreflective sheeting in the mold.
A preferred method of manufacturing a mold suitable for forming a retroreflective cube corner article includes providing an assembly having a plurality of laminae. A plurality of cube corner elements are formed in a working surface of the plurality of laminae. A portion of each of the plurality of the cube corner elements preferably extend across a portion of two or more laminae, wherein each of the cube corner elements comprises three approximately mutually perpendicular lateral faces that mutually intersect to define a cube corner element. A plurality of laminae are removed from the assembly containing less optically active portions of the cube corner elements.
A preferred master mold suitable for use in forming cube corner articles comprises a plurality of laminae disposed adjacent one another in an assembly. The respective laminae includes a microstructured working surface that includes a plurality of cube corner element segments. Each cube corner element segment corresponds to a portion of a fully formed truncated cube corner element. The laminae preferably have a thickness of about 0.05 millimeters to about 0.25 millimeters.
A preferred retroreflective sheeting modified to exhibit enhanced retroreflective efficiency at high entrance angles while maintaining high retroreflective efficiency at low entrance angles is also disclosed, including a substrate having a base surface and a structured surface opposite the base surface. The structured surface includes a plurality of optically opposing cube corner element segments of fully formed truncated cube corner elements. The cube corner element segments comprises primarily the optically active portion of a fully formed truncated cube corner element at a design entrance angle and orientation angle.